<?php
$rs=$db->Execute('select roles.rol_id as "rol_id", roles.rol_nombre as "rol_nombre" from roles, evento where roles.rol_evento = evento.evn_id && evento.evn_id = '.$_POST['evento'].''); ?>
        <select name="rol" class="bodyform" id="rol" onchange="refrescatabla(); return false;"  >
        <?php while(!$rs->EOF){ ?>
          <option value="<?php echo $rs->fields['rol_id'];?>" ><?php echo $rs->fields['rol_nombre']?></option>
          <? $rs->MoveNext(); ?>
          <? } ?>
        </select>
      </label>